Place the bacon and onions in a roasting pan and roast, stirring once or twice, until the onions are slightly golden and the bacon is semi-crisp, about 15 minutes. Transfer the onions and bacon, with all the drippings, to a bowl and set the pan aside. (Leave the oven on.) Add the bread cubes, garlic, rosemary, and parsley to the bowl and season with salt and pepper. You may need to moisten the mixture with a little olive oil or a splash of chicken broth. Let cool.

Meanwhile, prepare the pork: Preheat the oven to 450°F. Insert a sharp boning knife or other long thin knife into the center of one end of the roast and turn it in a circular motion to create a hole. Then insert a clean sharpening steel or the handle of a wooden spoon and push against the meat to create a larger space in the center of the pork loin. Continue until you’ve created a 1½-inch-diameter tunnel all the way through the pork.

Place the stuffing in a pastry bag without a tip and pipe the stuffing into the pork loin. Tie the pork loin into an even roll with kitchen twine. Season with salt and pepper and place in the roasting pan. Roast for 20 minutes.

Turn the oven down to 325°F and continue roasting for another 30 minutes, or until the internal temperature reaches 145° to 150°F on an instant-read thermometer. Allow the pork to rest on a rack set over a platter for 5 or 10 minutes.

Slice the pork 1 inch thick and arrange a couple of slices on each plate.